1. Field of the Invention
The present invention relates to an alcohol density sensor for detecting an alcohol content mixed in fuel such as gasoline. The alcohol density sensor is disposed in a fuel tank of an automotive vehicle.
2. Description of Related Art
An example of an alcohol density sensor is disclosed in JP-U-64-53957. An essential portion of the sensor is shown in FIGS. 4A and 4B attached hereto. The sensor includes a pair of electrodes 102 immersed in mixture fuel composed of gasoline and alcohol. The electrodes 102 for detecting an alcohol density in the mixture fuel are connected to a control unit 103 through wires 124 covered with insulator. A casing 104 for containing the alcohol density sensor therein is disposed in a fuel tank. A fuel pump 106 for supplying the mixture fuel to an internal combustion engine through a fuel supply passage 105 of the casing 104 is supported by a supporting member 106 fixed to a upper wall 107 of the fuel tank.
A pair conductor portions 111 integrally formed with the electrodes 102 is supported by an insulating resin member 113 contained in the casing 104. The pair of conductor portions 111 is connected to a pair of bare wires 121 of the insulator-covered wires 124 in the casing 104. The insulator-covered wires 124 are led out of the casing 104 through through-holes 114 and connected to the control unit 103. Small spaces between the through-holes 114 and the insulator-covered wires 124 are sealed with seal members 125. The seal members 125 prevent fuel pressurized by the fuel pump 101 from leaking to an outside of the casing 104 along the electrodes 102 and the conductor portions 111. Electric resistance in the mixture fuel becomes lower as an alcohol density in the mixture fuel becomes higher. Accordingly, the alcohol density can be detected by measuring the electric resistance between the pair of electrodes 102.
In the conventional sensor described above, a following problem is involved. Pressure of the mixture fuel in the casing 104 including the fuel supply passage 105 is higher than a pressure in the fuel tank (which is the same as atmospheric pressure), because the mixture fuel flowing through the fuel supply passage 105 is pressurized by the fuel pump 101. Therefore, the high pressure fuel may flow through a small gap between the bare wires 121 and the insulator covering the wire 124. There is a possibility that the mixture fuel leaked from the casing 104 may reach the control unit 103 that is positioned outside the fuel tank.